With the improving lumen efficiency of Light-Emitting Diode (referred to as LED for short) chip and package for illumination, Solid-State Lighting (referred to as SSL for short) including LED for general lighting is becoming an important application. Since standard 1 W LED is usually working with around 3.3V and 350 mA, for most applications, electronic drivers are needed to regulate the LED current. High frequency power electronic converters such as Buck converter, Fly-back converter or other converter with stepping-down topologies are often used in those electronic drivers.
For power electronic converter, Pulse Width Modulation (referred to as PWM for short) is the technique which can adjust the width of the conducting pulse of the power switch (for example, power semiconductor device), so as to control the amount of power sent to the load. PWM control could be realized with designated controller integrated circuit (referred to as IC for short) chips or with some micro-controllers. In most electronics converters with PWM control, the switching frequency is fixed. One problem with the fixed switching frequency is the high harmonics interference in power spectrum at multiples of the base frequency.
Electromagnetic interference (referred to as EMI for short), that is, the so-called radio frequency interference (referred to as RFI for short) is a disturbance that affects other electrical circuit due to either electromagnetic conduction or electromagnetic radiation emitted from an external source. There arc technical requirements for electronics products including all commercial/residential lighting products including EMI. Different countries or regions have their regulations for EMI, which means, the electronics products should generate less high frequency harmonics than required in the certain frequency range. To limit the EMI to the environment or to the AC line, input filter circuit is required to reduce high frequency harmonics in some applications, and this will increase cost and size of the system.
PWM control could be used in SSL for LED current regulating and/or for dimming control. Specifically, there will be two orders of PWM control. The first order of PWM control is by controlling the power semiconductor device switching to get constant LED driving current, wherein the switching frequency could be from 40 kHz to more than 1 MHz. The second order of PWM control is for dimming by switching operation the whole converter and LEDs, wherein the frequency is typically from 150 Hz to around 400 Hz. The frequency range of the second order of PWM control can help eliminate flickering effect of human eyes. Fixed frequency second order of PWM control will also have the high harmonics problem, and another problem is that, for some movie cameras with fixed recording frequency, fixed frequency regulation will cause flickering in the recorded video.
Electromagnetic conduction interference could be depressed by filter circuit (for example, inductors connected in series or capacitors in parallel). This is the most common solution for lighting sources with integrated electronic driver. However, input filter circuit will increase cost and size of the system. For some power electronic applications with PWM control, such as electrical machine drive or switch-mode power supply, Random PWM (referred to RPWM for short) has been used to distribute the EMI energy to wide frequency band, so as to reduce the harmonies amplitude and noise (Analysis and synthesis of randomized modulation schemes for power converters. Stankovic, A. M. Verghese, G. E.; Perreault, D. J.; Power Electronics, IEEE transactions on Volume 10, Issue 6, November 1995 Page(s):680-693). For LED lighting, since most state-of-arts designs do not have micro-controller to realize such complex control algorithm, drivers are still working at fixed switching frequency. With the increasing wattage level of the LED lighting systems and with integration of dimming function, noise and EMI will become more and more important for electronic design. However, there exist problems of larger circuit size, high EMI and LED flickering in the present technologies.
FIG. 1 is a circuit diagram of an example LED driving circuit according to an existing technology. As shown in FIG. 1, the LED driving circuit comprises capacitor C, free wheel diode FWD, inductor L, light emitting diode (or light emitting diode series) LED, and power switch PSW. The specific connection relations among those elements are shown in FIG. 1. The light emitting diode series LED is connected to the inductor L and the power switch PSW in series when the power switch PSW is turned on. The free wheel diode FWD will turn on to pass the inductor current when the power switch PSW is turned off. By regulating the duty cycle of the power switch PSW, the current of the light emitting diode series LED could be controlled. The switching frequency of the circuit could be from 40 kHz to more than 1 MHz. For the circuit with fixed switching frequency, FIG. 2 illustrates PWM driving signal and FIG. 3 illustrates the LED current waveform.
FIG. 4 is a diagram illustrating relations between output voltage and frequency under a control of the PWM driving signal shown in FIG. 2. As shown in FIG. 4, harmonies occurs at multiplies of the base frequency.
For PWM dimming, the duty cycle control is in low frequency of from 150 Hz to around 400 Hz. The power switch is still operating at the high frequency of kHz to MHz range, while the whole driving circuit is on and off at a low frequency. FIG. 5 shows simulated LED driving current waveform with PWM dimming according to the existing technology.
For the above technical problems, it is desired to provide a technique capable of reducing circuit size, decreasing EMI, and reducing flickering of the LED.